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KVM: x86/pmu: Snapshot and clear reprogramming bitmap before reprogramming

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Refactor the handling of the reprogramming bitmap to snapshot and clear
to-be-processed bits before doing the reprogramming, and then explicitly
set bits for PMCs that need to be reprogrammed (again).  This will allow
adding a macro to iterate over all valid PMCs without having to add
special handling for the reprogramming bit, which (a) can have bits set
for non-existent PMCs and (b) needs to clear such bits to avoid wasting
cycles in perpetuity.

Note, the existing behavior of clearing bits after reprogramming does NOT
have a race with kvm_vm_ioctl_set_pmu_event_filter().  Setting a new PMU
filter synchronizes SRCU _before_ setting the bitmap, i.e. guarantees that
the vCPU isn't in the middle of reprogramming with a stale filter prior to
setting the bitmap.

Link: https://lore.kernel.org/r/20231110022857.1273836-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2023-11-09 18:28:51 -08:00
parent b31880ca2f
commit 004a0aa56e
2 changed files with 30 additions and 23 deletions
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1
arch/x86/include/asm/kvm_host.h
View File

@@ -536,6 +536,7 @@ struct kvm_pmc {
#define KVM_PMC_MAX_FIXED 3
#define MSR_ARCH_PERFMON_FIXED_CTR_MAX (MSR_ARCH_PERFMON_FIXED_CTR0 + KVM_PMC_MAX_FIXED - 1)
#define KVM_AMD_PMC_MAX_GENERIC 6
struct kvm_pmu {
u8 version;
unsigned nr_arch_gp_counters;

52
arch/x86/kvm/pmu.c
View File

@@ -444,7 +444,7 @@ static bool pmc_event_is_allowed(struct kvm_pmc *pmc)
check_pmu_event_filter(pmc);
}
static void reprogram_counter(struct kvm_pmc *pmc)
static int reprogram_counter(struct kvm_pmc *pmc)
{
struct kvm_pmu *pmu = pmc_to_pmu(pmc);
u64 eventsel = pmc->eventsel;
@@ -455,7 +455,7 @@ static void reprogram_counter(struct kvm_pmc *pmc)
emulate_overflow = pmc_pause_counter(pmc);
if (!pmc_event_is_allowed(pmc))
goto reprogram_complete;
return 0;
if (emulate_overflow)
__kvm_perf_overflow(pmc, false);
@@ -476,43 +476,49 @@ static void reprogram_counter(struct kvm_pmc *pmc)
}
if (pmc->current_config == new_config && pmc_resume_counter(pmc))
goto reprogram_complete;
return 0;
pmc_release_perf_event(pmc);
pmc->current_config = new_config;
/*
* If reprogramming fails, e.g. due to contention, leave the counter's
* regprogram bit set, i.e. opportunistically try again on the next PMU
* refresh. Don't make a new request as doing so can stall the guest
* if reprogramming repeatedly fails.
*/
if (pmc_reprogram_counter(pmc, PERF_TYPE_RAW,
(eventsel & pmu->raw_event_mask),
!(eventsel & ARCH_PERFMON_EVENTSEL_USR),
!(eventsel & ARCH_PERFMON_EVENTSEL_OS),
eventsel & ARCH_PERFMON_EVENTSEL_INT))
return;
reprogram_complete:
clear_bit(pmc->idx, (unsigned long *)&pmc_to_pmu(pmc)->reprogram_pmi);
return pmc_reprogram_counter(pmc, PERF_TYPE_RAW,
(eventsel & pmu->raw_event_mask),
!(eventsel & ARCH_PERFMON_EVENTSEL_USR),
!(eventsel & ARCH_PERFMON_EVENTSEL_OS),
eventsel & ARCH_PERFMON_EVENTSEL_INT);
}
void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
{
DECLARE_BITMAP(bitmap, X86_PMC_IDX_MAX);
struct kvm_pmu *pmu = vcpu_to_pmu(vcpu);
int bit;
for_each_set_bit(bit, pmu->reprogram_pmi, X86_PMC_IDX_MAX) {
bitmap_copy(bitmap, pmu->reprogram_pmi, X86_PMC_IDX_MAX);
/*
* The reprogramming bitmap can be written asynchronously by something
* other than the task that holds vcpu->mutex, take care to clear only
* the bits that will actually processed.
*/
BUILD_BUG_ON(sizeof(bitmap) != sizeof(atomic64_t));
atomic64_andnot(*(s64 *)bitmap, &pmu->__reprogram_pmi);
for_each_set_bit(bit, bitmap, X86_PMC_IDX_MAX) {
struct kvm_pmc *pmc = kvm_pmc_idx_to_pmc(pmu, bit);
if (unlikely(!pmc)) {
clear_bit(bit, pmu->reprogram_pmi);
if (unlikely(!pmc))
continue;
}
reprogram_counter(pmc);
/*
* If reprogramming fails, e.g. due to contention, re-set the
* regprogram bit set, i.e. opportunistically try again on the
* next PMU refresh. Don't make a new request as doing so can
* stall the guest if reprogramming repeatedly fails.
*/
if (reprogram_counter(pmc))
set_bit(pmc->idx, pmu->reprogram_pmi);
}
/*